1. Field of the Invention
The present invention relates to an Nb.sub.3 Al multi-filamentary superconducting wire, and more particularly, to a compound superconducting wire serving as high magnetic field superconducting materials for superconducting magnets which can be used for nuclear fusion, superconducting energy storage or the like.
2. Description of the Background Art
A wire using compound superconductors is used as means attaining a high magnetic field which cannot be obtained with a wire using alloy superconductors such as NbTi. Among this compound superconductors, Nb.sub.3 Al particularly has a higher critical magnetic field of 30 T at 4.2K and superior distortion resistance characteristics. Nb.sub.3 Al is therefore expected as a high magnetic field superconducting material of the next generation.
Since Nb.sub.3 Al has poor ductility and cannot be directly worked like other compound superconductors, a wire is prepared by working a composite body including Nb, Al and a supplementary stabilizing material of copper or copper alloy into a wire having a final shape and thereafter producing Nb.sub.3 Al by diffusion reaction by heat treatment. Various methods of working a composite body into a wire such as a jelly roll method, a powder metallurgy method and a tube method have been developed according to Nb/Al composite forms.
Application of Nb.sub.3 Al to superconducting energy storage and nuclear fusion generating high electromagnetic force has been considered promising, taking advantage of its excellent distortion resistance characteristics.
Then, stabilization of a superconducting state and increase in capacity are required for a superconducting wire used for nuclear fusion and superconducting energy storage.
First, stabilization of a superconducting state of a superconducting wire requires a high residual resistance ratio, and in a nuclear fusion reactor, for example, requires a residual resistance ratio of at least 100. Thus, copper of high purity having a high residual resistance ratio has been used as matrix having filaments of superconductors embedded therein.
On the other hand, increase in capacity requires a multiple-stranded wire structure with at least several hundreds of strands. In such a multiple-stranded wire structure, however, a strand pitch is increased, whereby loss resulting from electromagnetic coupling between strands is increased. Then, in order to solve such a problem, a high resistance layer such as chromium has been provided at a surface of a wire in the case of a compound superconducting wire such as Nb.sub.3 Sn.
In order to realize both stabilization of a superconducting state and increase in capacity of an Nb.sub.3 Al multi-filamentary superconducting wire, we prepared a superconducting wire by combining the above-mentioned conventional examples to form a multiple-stranded wire structure using copper of high purity as a matrix and thereafter providing a high resistance layer thereon, but the following problems occurred.
More specifically, in preparation of an Nb.sub.3 Al multi-filamentary superconducting wire, heat treatment at a high temperature of at least 700.degree. C. is usually carried out for production of Nb.sub.3 Al. Consequently, in this heat treatment, elements contained in a high resistance layer such as chromium are diffused into a matrix of copper or copper alloy, causing reduction in residual resistance ratio.